1. Field of the Invention
The present invention relates to electronic semiconductor devices and specifically a structure of such a device comprising a heat dissipator and a plastic body joined to the dissipator and from which protrude the terminal leads of the device.
2. Description of the Prior Art
As known, integrated circuits are provided on semiconductor chips and require for their connection to an outside electrical circuit special support structures for containment and electrical connection.
A typical structure suited for the purpose consists essentially of a plastic body (termed package) enclosing the chip which is connected by means of thin metal wires welded on special metallized areas provided on its surface to corresponding electrical conductors--or terminal leads--emerging from the plastic body.
In the case of integrated power circuits, i.e. devices designed for operation with high currents and, therefore, capable of producing heat, said structures also comprise a metallic dissipator element through which the chip fastened thereon can transfer to the exterior the heat produced during its operation.
For the fabrication of these packages, on the dissipator is mounted a frame consisting of metallic strips still joined together by interconnection lengths and designed to become the terminal leads of the device. Said framework is made by blanking from a copper sheet and exhibits holes opposite special columns integral with the dissipator.
The free ends of these columns are riveted at the time the framework is positioned and constrained on the dissipator to ensure electrical connection between the dissipator and the frame. Typically the unit formed by the dissipator and the metal framework joined together is repeated a certain number of times to form a continuous metallic strip. This structure is termed "leadframe".
Then the chip is fastened on the dissipator by welding with a low boiling-point alloy, e.g. lead-tin alloy, or by cementing with a suitable adhesive, e.g. an epoxy glue.
Thin gold wires are welded on one end to the peripheral terminals (pads) of the chip and on the other end to the internal end of the metallic strips by a process termed "thermosonic" which provides the simultaneous application of heat and ultrasounds.
In certain cases it is required that at least one of the pads of the chip, normally connected to the electrical ground of the circuit, be electrically connected to the dissipator. The connection is established as described in European patent application of the same applicant no. 92115760.8 published 9 Jun. 1993 under no. 0545007 with a thin gold wire fastened on one end to the metallized area of the chip and at the other end to special pads made on the surface of the metal framework.
Subsequently the assembly is mounted in a special mould in which is injected a plastic resin in liquid state. After polymerisation of the resin there is obtained a structure comprising a solid plastic body incorporating the above described elements with the exception of one face of the dissipator, part of the terminal leads of the device, and the interconnection lengths between the terminal leads. These interconnection lengths are then eliminated by blanking and there is obtained thus the finished device.
Normally the leadframes are produced in a place different from where the entire device is assembled or where processing steps are performed, discussed above, and which lead to blanking of the interconnection lengths between the terminal leads and between the leadframes themselves.
These leadframes are inserted by stacking a certain number of identical parts in special containers designed for transportation and are shipped to the place where processing of the device continues. In these containers the leadframes are rested on each other and thus the top face of each leadframe is in contact with the bottom face of the leadframe above.
As the surfaces of some pads designed for electrical connection of some areas of the semiconductor chip with the dissipator are metallized with silver it may happen that during packing and transportation operations, because of mutual rubbing of the leadframes, said surfaces are partially damaged. This may cause defects during the subsequent processing steps and reduce considerably the reliability of the welding.
A contrivance used to protect the metallized areas on the surface of the leadframes consists of placing between one leadframe and the next protective spacers made of shock-resistant material.
A first shortcoming encountered when using this type of packing is the need to remove the spacers one by one before loading the leadframes in the assembly machines of the semiconductor chips and welding of the connecting thin gold wires. In addition, these spacers having a certain thickness, the dimensions of the containers used for transportation are much greater than the actual volume occupied by the leadframes.
It is therefore an object of the present invention to conceive, for electronic semiconductor devices, a leadframe having structural and functional characteristics such as to overcome the shortcomings encountered in the prior art.
The characteristics and advantages of the leadframe in accordance with the present invention are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the annexed drawings.